Since free radical polymerizations, once initiated—whether intentionally or unintentionally—are strongly exothermic, the polymerization may accelerate, on inadequate removal of the heat of reaction, by self-induced heating of the reaction mass such that it gets out of control and critical conditions—including up to explosion of the reaction vessel—occur.
If the abovementioned heat removal takes place in an inadequate manner in free radical polymerizations, there is a danger that the polymerization will be so vigorous that the vessel containing the polymerization mixture will explode if the runaway polymerization (for example, free radical mass, solution, emulsion or suspension polymerization of compounds (monomers) having at least one ethylenically unsaturated group) is not counteracted.
However, such an effective countermeasure is also required, in particular in the case of unintentionally initiated free radical polymerizations. Unintentionally initiated free radical polymerizations may occur, for example, during the storage and/or transport of substances containing monomers, since both heat and light or undesired free radicals can initiate the free radical polymerization of monomers. It is true that an attempt is usually made to prevent such unintentional free radical polymerizations by adding small amounts (as a rule up to 1000 ppm by weight) of free radical polymerization inhibitors (free radical acceptors, polymerization inhibitors) to the monomers. However, their inhibiting effect must not be too pronounced since otherwise they would have to be separated off before subsequent use of the monomers for free radical polymerization purposes. However, free radical polymerization initiators can usually predominate over a moderately inhibiting effect, as possessed, for example, by the monomethyl ether of hydroquinone (MEHQ), and it is for this reason that MEHQ is a storage and/or transport stabilizer particularly frequently used for monomers. However, experience has shown that, even in the case of monomers stabilized with storage and/or transport stabilizers, an unintentional free radical polymerization of said monomers cannot be completely ruled out. The latter applies in particular when the monomers are (meth)acrylic monomers and/or styrene, which particularly readily undergo free radical polymerization.
The term (meth)acrylic monomers is understood here to mean substances which comprise acrylic acid or methacrylic acid or derivatives thereof, for example, acrolein, methacrolein, acrylic acid, methacrylic acid and/or esters of the two acids just mentioned. In this description, (meth)acrylic is generally used as an abbreviation for acrylic and/or methacrylic.
Especially substances which comprise at least 90% by weight of (meth)acrylic monomers and/or styrene are at risk with regard to an unintentional free radical polymerization (this also applies when polymerization inhibitors are added as a preventive measure). This applies in particular when such substances are exposed to extreme external conditions during transport and/or during storage (for example, extremely high temperatures during the transport by ship through the various climatic zones (e.g. across the equator), as is the case, for example, for transport from Europe to Southeast Asia, or extremely low temperatures, as in the case of storage in outdoor tanks in northern countries. In particular, low temperatures are not without risks since they can in extreme cases lead to a partial or complete crystallization of the monomers. The latter usually results in separation of monomers and stabilizer (purification by crystallization), which may lead to the presence of unstabilized regions of monomers and subsequent melting for a certain duration, which regions can with high probability be the starting point of an unintentional free radical polymerization.
For safe transport and/or safe storage of monomer—containing substances, there is therefore a need for a process which is capable of very rapidly terminating an unintentionally initiated free radical polymerization of the monomers. Such a process is also required, however, for stopping runaway unintentional free radical polymerizations immediately.
An effective countermeasure to such a free radical polymerization becoming uncontrollable has already been described in the literature (see, for example, Process Safety Progress 1993 (Vol. 12 No. 2), pp. 111-114, WO 99/21893 A2 and DE 199 18 970 A1).
Solutions of known inhibitors of free radical polymerization [e.g. phenothiazine or hydroquinone monomethyl ether (MEHQ)] are used for this purpose.
Whereas in the journal article in Process Safety Progress no reference is made to possible solvents, the two patent documents mentioned above describe the use of N-methylpyrrolidone and/or N-ethylpyrrolidone.
The N-alkylpyrrolidones mentioned are good solvents for phenothiazine and/or MEHQ and solutions thereof are stable at low temperatures down to −11° C. Moreover, their water-miscibility is seen as an advantage since they are also applicable to aqueous-based systems in this form.
However, alkylpyrrolidones are subject to labelling due to their teratogenic effect and toxicity if swallowed. There is, therefore, considerable interest in alternative formulations of the inhibitors with (water-miscible) organic solvents which are toxicologically harmless and ideally not subject to labelling. The literature cited above gives no indication of such solvents.
It has been found in some experiments that phenothiazine is only poorly soluble in conventional organic solvents (e.g. the solubility in methanol, ethanol, isopropanol, toluene, cyclohexane or n-hexane is less than 2-3 percent by weight). Only in the N-methylpyrrolidone already mentioned and in the chemically and toxicologically related dimethylformamide could concentrated solutions of 30 percent by weight or more be prepared.
Tetrahydrofuran and acetone could also be identified as good solvents. Unfortunately these solvents have low boiling points and flashpoints which render transport, storage and handling of the solutions more difficult.
The object of the present invention, therefore, was to provide a method for immediately terminating free radical polymerizations in the most advantageous and most broadly applicable manner.